Ink jet printers have print heads that operate a plurality of ejection jets from which liquid ink is expelled. The ink may be stored in reservoirs located within cartridges installed in the printer, or the ink may be provided in a solid form and then melted to generate liquid ink for printing. In these solid ink printers, the solid ink may be in either pellets, ink sticks, granules or any other shape. The solid ink pellets or ink sticks are typically placed in an “ink loader” that is adjacent to a feed chute or channel. A feed mechanism moves the solid ink sticks from the ink loader into the feed channel and then urges the ink sticks through the feed channel to a heater assembly where the ink is melted. In some solid ink printers, gravity pulls solid ink sticks through the feed channel to the heater assembly. Typically, a heater plate (“melt plate”) in the heater assembly melts the solid ink impinging on it into a liquid that is delivered to a print head for jetting onto a recording medium.
A typical inkjet printer uses one or more printheads. Each printhead typically contains an array of individual nozzles for ejecting drops of ink across an open gap to a receiving member to form an image. The receiving member may be recording media or it may be a rotating intermediate imaging member, such as a print drum or belt. In the print head, individual piezoelectric, thermal, or acoustic actuators generate mechanical forces that expel ink through an orifice from an ink filled conduit in response to an electrical voltage signal, sometimes called a driving signal. The amplitude, or voltage level, of the signals affects the amount of ink ejected in each drop. The driving signal is generated by a print head controller in accordance with image data. An ink jet printer forms a printed image in accordance with the image data by printing a pattern of individual drops at particular locations of a pixel array defined for the receiving medium. The locations are sometimes called “drop locations,” “drop positions,” or “pixels.” Thus, the printing operation can be viewed as the filling of a pattern of drop locations with drops of ink.
Some ink jet print heads, such as phase change ink jet print heads, utilize inks that have melting points of 80° C. and higher. With many of these inks, optimal jetting occurs at significantly higher temperatures, such as 120° C. and above. Consequently, during printing the ink jets and other print head components must be maintained at or above these elevated jetting temperatures. The temperature of the ink reservoirs supplying liquid ink to the ink jets must also be maintained at or near the required jetting temperatures.
Prolonged use of an ink jet print head at elevated temperatures can alter print head performance and accelerate thermal stress or aging of the print head components. Thermal aging, also known as drift, can result in image degradation due to performance variations. For example, the drop mass of ejected ink drops can vary as the print head components are thermally conditioned over time. Variations in drop mass from nozzle to nozzle of a print head or from print head to print head in a multiple print head system may result in result in banding or streaking of a printed image or non sharp edges to lines or shapes due to positional errors resulting from drift.
To reduce ink drop mass variations due to thermal aging of the print heads of an ink jet printer, previously known systems implemented an open loop routine in which a controller altered the voltage level of the driving signals for the print head over time at a predefined rate that was designed to compensate for the drift of a generic print head. The variability of the drift behavior between different print heads in a printer, however, may be significant. Therefore, adjusting the driving voltages of the print heads in this manner may eventually result in print heads outputting drops at different drop masses.